1. Field of the Invention
This invention relates to (1) stable aqueous solutions (or dispersions) containing siloxanes, some of which siloxane precursors, namely the corresponding silanes, are new and novel, (2) a process for treating a cellulosic substrate, particularly a wood substrate with said stable aqueous solutions to provide said wood with long-term durability against weathering, and (3) the treated cellulosic substrate, particularly a wood substrate.
2. Description of the Prior Art
Silanes containing hydrolyzable groups, such as methoxy, for many years were believed to be unstable in the presence of a quantity of water near or even in slight excess of the amount stoichiometrically required to substantially fully hydrolyze the same. Only silanes with special structural features, i.e., aminopropylsilanes, were known to form stable aqueous solutions, that is, aqueous solutions that would not tend to form gels or precipitates upon standing over a long period of time, but these had neither hydrophobic or water-solubilizing moieties contained in the silanes used in the preparation of the stable aqueous solution claimed herein. Thus, low molecular weight silanes have been commercially employed at high dilution (generally less than about five weight percent in water) for treating inorganic fibers, as adhesion promoters to chemically bond said fibers to an organic polymer, plastic or resin, for example, as described in U.S. Pat. No. 3,973,057 to Clannin.
Part of the reason for aqueous solutions of siloxanes of low dilution may be related from the fact that it is well known that trialkoxy silanes will hydrolyze stepwise in water to give the corresponding silanols, which ultimately condense to siloxanes. See Silane Coupling Agents, by E. P. Plueddemann, Plenum Press, New York, N.Y., 1982, pages 32 and 33. It would be expected that since condensation results in the production of extremely large molecular siloxane entities, the latter would be expected to be insoluble or only partially soluble in water and gel formation or precipitation would occur. Therefore, as seen, for example, in U.S. Pat. No. 4,913,972 to Grunewalder et al and U.S. Pat. No. 4,386,134 to Puhringer, they use partially hydrolyzed silanes or silanes in organic solvents and speak rather vaguely of the use of water as a solvent therefor. Plueddemann, in his book referred to above, in fact is quite careful in his assessment of aqueous solutions containing siloxanes. Thus, on page 51, Section 3.22, he states that solubility of hydrolyzates containing silane triols decreases both as the hydrocarbon content of the precursor silane increases as silanols condense to oligomer silanols, but on page 58, Section 3.3.1, he states that certain amino organofunctional trialkoxysilanes, without defining any specific compounds, are readily soluble in water to give solutions of unlimited solubility without defining what such amounts of siloxanes are solubilized therein. He then says quite generally on page 60, Section 3.3.1, that if the aminoalkyl on silicon is sufficiently hydrophobic, the polysiloxanes retain water solubility, and then goes on to further solubility considerations in water that apparently are not inconsistent with the reluctance of those skilled in the art to dissolve organosilanes in water in high concentrations.
Applicants have found that they can introduce specific silanes containing both water-solubilizing and hydrophobic moieties, which silanes will be defined more specifically hereinafter, some of which are new and are claimed in our copending application identified above, in water in amounts well in excess of the amount stoichiometrically required to substantially fully hydrolyze the same, for example, concentrations from about five to about 60 percent, or even higher, to obtain aqueous stable solutions, that is, aqueous solutions that will not tend to form gels or coagulate, for a period of at least about 20 days, even upto about 300 days, or even more. Such stability is obviously highly desired, since it permits shipment of these products with a minimum freight expense, because of the high concentration of silanes therein, and also permits storage thereof over a long period of time. These aqueous solutions are, of course, not moisture sensitive as are compositions containing their organosilane precursors.
The novel stable aqueous solutions containing siloxanes, defined above, can be used to treat any cellulosic material, such as wood or composites prepared therefrom, such as defined in U.S. Pat. No. 4,913,972 of Grunewalder et al, referred to above, paper, etc., but are eminently suitable for treating wood to improve its durability to natural weathering.
Solid wood substrates for use in outdoor applications, such as in exterior building materials of various types, typically are protected from the effects of weathering by painting the wood with a conventional paint or by staining the wood with, for example, a pigmented opaque stain. While coatings which are opaque, or substantially so, can afford good protection against weathering, particularly against the combined effects of moisture and ultraviolet light, they do not allow the natural beauty of the wood, such as the grain of the wood, to be seen. It has long been desirable to provide a coating system for exterior wood which is transparent, or essentially so, and yet still affords excellent protection against weathering. However, this object has been elusive with respect to achieving relatively long term exterior durability in a transparent coating system for wood. For example, wood which has been coated with a conventional, clear coating such as a conventional air dry urethane, alkyd or spar varnish often exhibits signs of deterioration in both the coating and, more importantly, in the underlying wood itself, in as little as one year from the time of application.
It is observed in the article, "Microscale Effects of Ultraviolet Irradiation and Weathering on Redwood Surfaces and Clear Coatings," Journal of Paint Technology, Vol. 41, No. 531, page 275 ff, (April, 1969) that,
"Most conventional clear coatings strongly absorb ultraviolet radiation, which leads to their rapid degradation. Then degradation of the wood surface beneath may follow. When clear coatings are used that are transparent to ultraviolet radiation, the coating may be very stable, but the wood surface may then be rapidly degraded. The photodegradation of the wood surface, therefore, may be an important factor in the relatively short life of clear coatings that transmit all or only a portion of the ultraviolet wavelengths of solar radiation." PA0 "Any wood exposed to sunlight and rain will in time lose its original colour. The change is due partly to loss of water-soluble extractives, which occurs quite rapidly, but mainly to the breakdown of lignin and other components in the wood by ultra-violet components in sunlight, and subsequent removal of the breakdown products by rain; and further with respect to clear varnishes as follows. PA0 "Fully exposed varnish often has a short life, principally because of the tendency of most types to embrittle by weathering, so that in time they disintegrate under the stresses imposed by a wood substrate. Ultra-violet light can pass through the film and bleach or degrade the underlying wood surface. Many cases of premature varnish failure can be traced to detachment of this degraded wood surface layer; this is often brought about by water getting behind the film where end grain is exposed, for example at joints or unstopped nail holes, or as a result of damage." PA0 "These shortcomings are common to all of the varnishes commonly used. Polyurethanes of the moisture-curing and two-pack types give good service indoors but the stresses within such a [sic] strong coatings are sufficient to cause peeling when the outermost layer of wood under the film become degraded; the remnants of the coating are difficult to strip either mechanically or chemically. Unless there are exceptional requirements for toughness and abrasion resistance, polyurethanes are therefore best avoided outdoors. Marine or yacht varnishes have also not been found satisfactory under conditions of full exposure on buildings." PA0 "The addition of colorless UV light absorbers to clear finishes has found only moderate success to help retain the natural color and original surface structure of wood . . . Opaque pigments found in paints and stains generally provide the most effective and longlasting protection against light . . . Even when using relatively durable, clear, synthetic resin varnishes, the weatherproof qualities of the wood-varnish system are still limited because UV light penetrates the transparent varnish film and gradually degrades the wood under it . . . Eventually, the varnish begins to flake and crack off, taking with it fibers of the wood that have been degraded photochemically . . . " [citations to references deleted].
Moreover, it was observed in the aforesaid article, that a silicone resin which "was known to transmit considerably more of the ultraviolet radiation that the two varnishes" (a phenolic varnish and a polyurethane varnish), offered little if any protection from degradation upon exposure to a source of ultraviolet light.
It is taught in the article, "Natural finishes for exterior timber," Pigment and Resin Technology, April, 1986, page 10 ff, that
In the article, "Chemistry of Weathering and Protection," The Chemistry of Solid Wood, Capter 11, published 1984 by the American Chemical Society, page 435 ff, it is taught that,
Organosilanes have been used in the treatment of wood, for example, in U.S. Pat. No. 4,913,972 to Grunewalder et al and U.S. Pat. No. 4,386,134 to Puhringer, referred to above, as well as in U.S. Pat. No. 4,429,082 to Lee et al. However, Grunewalder et al use partially hydrolyzed organosilanes and organosilicates with organic solvents as carriers therefor, specifically toluene in the working examples. After listing many organic solvents as being useful as carriers, Grunewalder et al state that some of the organosilicon-containing compounds useful therein can be used in aqueous, rather than organic solvent borne wood treatment compositions, but neither identifies the silanes, nor the stability nor the concentration thereof in an aqueous solution. Puhringer says he impregnates wood with a silane in the form of a solution, suspension or emulsion, the continuous phase being a gas, such as air or water vapor, liquids, such as water, alcohols, such as lower alkanols with 1 to 4 carbon atoms, mixtures of water and such alcohols, light petrol, naphtha or other petroleum products, ligroin, esters, benzene, toluene, xylene, hydrocarbons and chlorinated hydrocarbons, tars, cresot oils, etc., but in the specific examples shows only the use of aqueous ethanol and does not comment on the stability of the treating solutions. Lee et al use organic solvents only, if a carrier is to be used.
The compounds of the above mentioned references also contain hydrolyzable groups, such as alkoxy groups, attached to the silicon atom. This fact renders these compounds moisture sensitive. What this means is that open containers of the silicon-containing compounds, even if dissolved in organic solvents, will react with the moisture from the air and hydrolyzed and further condense to form polymeric species. The result is that these materials are, from a practical standpoint, very difficult to use successfully. The open containers form skins on the top of the liquid, form precipitates which are particulates which tend to clog spray equipment or if applied to a surface, form rough coatings due to the particulates. The open container may even cause the components therein to gel or solidify due to hydrolysis and condensation reactions due to moisture exposure.
In view of the above, it would have been expected that since hydrolysis and condensation of the silanes would produce, in any event, large molecules, they would not be suitable for use in treating a cellulosic porous substrate, such as wood, for little or no penetration would be expected and yet herein, as will be shown hereinafter, the aqueous solutions are not only stable but can be easily applied to wood and the wood so treated will exhibit long-term durability against weathering.
The coating industry is desirous of using as little as possible of organic carriers, particularly volatile organic compounds (VOC), and, in fact carriers other than the above. Organic solvents can be toxic, they can be lost to the ambient surroundings, causing pollution problems, and they can be expensive and sometimes in short supply. For these reasons water-based coatings are desirable when they can be used in a coating operation without loss of performance of the coating medium, since water is relatively cheap, non-toxic, does not result in environmental problems, etc. Accordingly, the water-based stable solutions used herein for treating wood result in a treated wood having durability against weathering that is as good, or even better, than wood with similar type treating agents in an organic carrier.